The polymerization of 6-aminocapronitrile to form nylon polymer is disclosed in Greenewalt U.S. Pat. No. 2,245,129, and Curatolo et al. U.S. Pat. No. 4,568,736.
When 6-aminocapronitrile is produced by partial hydrogenation of adiponitrile, hexamethylenediamine and tetrahydroazepine, i.e. the latter compound represented by the formula: ##STR1## also coproduced. The hexamethylenediamine is easily removed from the mixture by simple distillation, but the tetrahydroazepine (hereinafter sometimes referred to as THA) is not easily separated. The presence of THA in the 6-aminocapronitrile (hereinafter sometimes referred to as 6-ACN) that is to be polymerized limits the molecular weight of the polymer and causes color and branching in the polymer. It is, therefore important that THA be removed from the 6-ACN before polymerization.
It is an object of the present invention to provide a simple and efficient method of obtaining 6-ACN that is free from THA.
The use of hydrides to prevent color formation in ethanol amines is disclosed in Moore U.S. Pat. No. 3,159,276, and the use of hydrides to reduce the color of ethanol amines is disclosed in Glew et al. U.S. Pat. No. 3,207,790. The use of sodium borohydride to reduce carbon/nitrogen double bonds in hetrocyclic compounds is also known and summarized in "Sodium Borohydride Digest" (1989) pages 24-26, Morton International, Specialty Chemicals Group.